Method for producing printed wiring boards

ABSTRACT

A method for producing printed wiring boards comprises the steps of perforating through holes at predetermined positions in an adhesive insulator sheet, filling the through holes with a conductive material such as a conductive paste or metal balls, transferring conductive wiring patterns that have been formed on surfaces of releasable supporting sheets onto the surfaces of the adhesive insulator sheet by heat and pressure. Simultaneously, interlayer via-connections are performed by means of the conductive material filled into the through holes.

BACKGROUND OF THE INVENTION

The present invention relates to a method for producing printed wiringboards (PWBs), especially double-side PWBs or multilayer PWBs.

Recently, a low cost multilayer PWB that enables high-density mountingof semiconductor chips such as LSIs has been desired for industrialmachines as well as home electronic appliances. It is important for suchmultilayer PWBs to provide high quality electric connections betweenplural layers of wiring patterns formed at a fine spacing or pitch.

The conventional PWB that is produced by drilling, etching and plating acopper-foil laminated board cannot satisfy the above-mentioned needanymore for sophisticated electronic equipment having a number offunctions. To solve such a problem, some methods are under developmentfor producing PWBs that have a new structure and a high density ofwiring.

One of the methods is a recent technique for forming a fine pattern thatcan be applied to a high-density surface mounting. This method forproducing PWBs utilizes a plating technique and a transferring techniquefor the wiring pattern. Two metal sheets are prepared, each of which hasa surface with a wiring pattern formed by electroplating of copper. Asemi-hardened resin sheet such as a prepreg is placed between the wiringpatterns of the metal sheets. Heat and pressure are applied to the outersurfaces of the metal sheets. Thus, the copper wiring patterns aretransferred from the surfaces of the metal sheets to the surfaces of theresin sheet. After removing the metal sheets, through holes are formedby drilling in the resin sheet, and copper plating is performed insidethe through holes to connect the wiring pattern on one side with that ofthe other side electrically (Naoki Hukutomi et al. “Development of FinePattern Wiring Technique”, The Institute of Electronics, Information andCommunication Engineers, C-II, Vol. J72-C-II, No. 4, PP243–253, 1989).This method provides a line width and a line space of 20 microns each.

There is another technique called “ALIVH” (a trademark of MatsushitaElectric Industrial Co., Ltd.), that is a resin-based multilayer PWBhaving an inner via hole (IVH) structure for all layers. In thismultilayer PWB, a conductive material is filled in the inner via holesinstead of copper plating inside the through holes that is a majormethod for electric connection between layers of a multilayer PWB in theprior art. This “ALIVH” PWB thus improves the reliability of theelectric connection between layers, and facilitates forming inner viaholes under lands for mounting components or between any layers (U.S.Pat. Nos. 5,346,750 and 5,481,795).

An example of the method for producing the “ALIVH” PWB is explainedbelow, referring to FIGS. 5A–5F that show cross sections in theproducing process. As shown in FIG. 5A, via holes 502 are perforated byusing a laser beam machine at predetermined positions in an adhesiveinsulator sheet 501 that comprises an aramid-epoxy prepreg made of anon-woven aramid sheet impregnated with an epoxy resin. Then, as shownin FIG. 5B, the via holes 502 are filled with a fluid conductive paste503. Then, as shown in FIG. 5C, the adhesive insulator sheet 501 withvia holes filled with the conductive paste is placed between copperfoils 504, and heat and pressure are applied to the outer surfaces ofthe copper foils. Thus, the adhesive insulator sheet (the prepreg) 501and the conductive paste 503 are hardened, the copper foils 504 adhereto the surfaces of the adhesive insulator sheet 501, and electricalconnections are formed between the copper foils by the conductive paste503 packed into the via holes 502. The copper foils 504 are etched by aconventional photolithography method to form wiring patterns 505 a, 505b. Thus, a double-side PWB 506 is obtained as shown in FIG. 5D.

In the next step shown in FIG. 5E, the double-side PWB 506 is used as acore, and on both sides of the core PWB 506 other adhesive insulatorsheets 501 a and 501 b are placed with proper registration. Theseadhesive insulator sheets 501 a, 501 b have been made previouslyaccording to the step shown in FIG. 5B, and each of them has via holesfilled with the conductive paste at predetermined positions. On theouter surfaces of the adhesive insulator sheets 501 a and 501 b, copperfoils 507 a and 507 b are placed. Heat and pressure are applied to bothouter surfaces of the copper foils 507 a, 507 b for lamination. Then,similarly to the step of FIG. 5D, the outer copper foils 507 a, 507 bare etched by the photolithography method. Thus a four-layer PWB isobtained having the outer wiring patterns 508 a, 508 b as shown in FIG.5F. This method for producing PWBs enables via-connections (electricconnections between layers) with very small via holes since via holesare formed by a laser beam and filled with the fluid conductive pastefor the electric connection.

However, in the above-mentioned transferring technique of the wiringpattern, there is a limit for reducing the size of through holes sincethey are perforated by machining. On the other hand, in the above“ALIVH” PWB, there is a limit on the fineness of patterns with respectto pattern density such as a line pitch and a line width since the outerand inner copper patterns are formed by the conventionalphotolithography method. These limits are obstacles to producing highdensity mounting of surface mount components, especially smallelectronic components such as recent chip components or LSI bare chips.

SUMMARY OF THE INVENTION

The present invention provides a method for producing fine pattern PWBsthat enables high density mounting of components by combining theadvantage of the conventional transferring method of wiring patterns andthe advantage of the conventional “ALIVH” structured multilayer PWB.

The method for producing PWBs according to the present inventioncomprises the steps of perforating through holes at predeterminedpositions in an adhesive insulator sheet, filling the through holes witha conductive material, forming conductive wiring patterns on surfaces ofreleasable supporting sheets, and transferring the conductive wiringpatterns from the surface of the releasable supporting sheets ontosurfaces of the adhesive insulator sheet so as to form the wiringpatterns on the surfaces of the adhesive insulator and perform electricconnections between the wiring patterns of plural layers.

According to the present invention, PWBs can be produced with finepatterns defining very fine wiring pitches and electric connections byvery small via holes. In addition, such fine PWBs can be produced at alow cost since the method according to the present invention is simplecompared with other conventional methods.

Another method according to the present invention uses the PWB producedby the method mentioned above as a core. Second adhesive insulatorsheets are prepared that have through holes filled with a conductivematerial. These second adhesive insulator sheets are placed on surfacesof the core PWB. On the outer surfaces of the second adhesive insulatorsheets, second releasable supporting sheets are placed, whose surfacesfacing the second adhesive insulator sheets are provided with secondconductive wiring patterns. The second conductive wiring patterns aretransferred from the surfaces of the second releasable supporting sheetsonto the surfaces of the second adhesive insulator sheets so as to formsurface wiring patterns and perform electric connection between thesurface wiring patterns and the inner wiring patterns. Thus, multilayerPWBs with fine patterns can be produced inexpensively. By repeating thesteps mentioned above, PWBs having more layers can be produced easily.

Another method according to the present invention uses a double-side ormultilayer PWB produced by a conventional method as a core. Adhesiveinsulator sheets that have via holes filled with a conductive materialare placed on surfaces of the core PWB. Conductive wiring patterns onsurfaces of releasable supporting sheets are transferred onto outersurfaces of the adhesive insulator sheets so as to form surface wiringpatterns and perform electric connection between the surface wiringpatterns and the inner wiring patterns. Also in this method, PWBs havingmore layers can be produced easily by repeating the steps mentionedabove.

In each method mentioned above, it is preferable to use a fluidconductive paste as the conductive material to be packed into thethrough holes. The use of the fluid conductive paste enables reliableelectric connections with very small via holes.

It is preferable that conductive sheets are used as the releasablesupporting sheets and the conductive wiring patterns are formed byelectroplating after forming resist films on surfaces of the releasablesupporting sheets. Finer patterns can be obtained by electroplatingafter printing fine resist pattern than etching conductive layer.Furthermore, production cost may be reduced because of less waste ofconductive material.

Alternatively, the conductive wiring pattern can be formed by printing aconductive paste on the surface of the releasable sheet. This methodenables formation of conductive wiring patterns at a low cost.

It is also preferable that insulated and via-connected multilayer wiringpatterns are formed in the step of forming wiring patterns on thesurface of releasable supporting sheets. This produces PWBs havingplural inner wiring patterns at a single transferring.

It is preferable to use a semi-hardened resin insulator as the adhesiveinsulator sheet. This semi-hardened insulator sheet becomes hardenedcompletely when the heat and pressure are applied for transferring thewiring pattern from the surface of the releasable supporting sheet ontothe surface of the adhesive insulator sheet. Thus, a large adhesionstrength is obtained between the conductive wiring pattern and thehardened insulator sheet.

The adhesive insulator sheet is preferably made of a porous andcompressible semi-hardened material. This adhesive insulator sheet iscompressed when the heat and pressure are applied for transferring thewiring pattern from the surface of the releasable supporting sheet ontothe surface of the adhesive insulator sheet. Simultaneously, theconductive material in the via holes is compressed so that reliablevia-connections with good conductivity can be obtained.

More preferably, the adhesive insulator sheet is made of a prepreg thatis a non-woven aramid sheet impregnated with an epoxy resin. Thismaterial has ideal properties of transferring ability andcompressibility. Since this material has light weight and its thermalexpansivity is as low as a ceramic PWB, PWBs of high utility with lowdielectric constant and high resistance to heat are obtained.

In each method for producing PWBs mentioned above, it is preferable touse heat and pressure for transferring the conductive pattern from thesurface of the releasable supporting sheet onto the surface of theadhesive insulator sheet and hardening the adhesive insulator sheetcompletely. The heat and pressure make the conductive wiring patterntransfer and adhere securely to the surface of the adhesive insulatorsheet. Thus, the PWB produced by this method has a fine wiring patternand high mechanical strength due to the promoted cure of the adhesiveinsulator.

The PWB according to the present invention comprises an adhesiveinsulator sheet having through holes filled with a conductive material,a conductive wiring pattern that is formed on the surface of theadhesive insulator sheet by the transferring method and is connectedwith the conductive material in the via holes, wherein the conductivewiring pattern is embedded in the adhesive insulator sheet so as to forma flat surface. Such a PWB is suitable for flip mounting of LSI chipssince the surface of the PWB is flat.

It is preferable that a width of the conductive wiring pattern formed onthe adhesive insulator sheet is smaller than a diameter of the via holeat least where the conductive wiring pattern is overlaid on the viahole. This configuration can relieve a maximum allowable limit ofpattern deviation, which is critical for PWBs with fine patterns.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A–1E are cross sections showing a process for producing a PWBaccording to a first embodiment of the present invention;

FIG. 2A shows an allowable deviation of a wiring pattern from a via holeaccording to the present invention;

FIG. 2B shows an allowable deviation of a wiring pattern from a via holein the prior art;

FIGS. 3A–3C are cross sections showing a process for producing a PWBaccording to a second embodiment of the present invention;

FIG. 4 is a cross section of a releasable supporting sheet used forproducing a PWB according to a third embodiment of the presentinvention; and

FIGS. 5A–5F are cross sections showing a process for producing a PWB inthe prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

A first embodiment of the present invention is illustrated in FIGS.1A–1E, which are cross sections showing a process for producing a PWB.

In FIG. 1A, numeral 111 is a releasable supporting sheet, preferablymade of a metal sheet such as a stainless steel having properly roughsurface. In the step of FIG. 1A, a patterned resist film 112 is formedon the rough surface of the releasable supporting sheet.

In the step of FIG. 1B, a wiring pattern 113 is formed by electroplatingon the rough surface of the releasable supporting sheet except the areawhere the resist film has been formed, and then the resist film isremoved.

In FIG. 1C showing the next step, numeral 114 is an adhesive insulatorsheet, preferably made of an aramid-epoxy prepreg that is a non-wovenaramid sheet impregnated with an epoxy resin. The aramid-epoxy prepregis a porous semi-hardened sheet having compressive and adhesiveproperties suitable for forming fine pattern and via-connection of highreliability according to the present invention. It also has an advantagein that the electric resistance of the via-connection becomes small.

The adhesive insulator sheet 114 has small through holes 115, which arefilled with a conductive material 116. The diameter of the through holesis approximately 150 microns when using a carbon dioxide laser, and canbe 30–50 microns by using an excimer laser. A fluid conductive pastewhose viscosity is 1,000–3,000 poise is suitable for the conductivematerial 116. Such a conductive paste contains copper powder, resin andhardener.

As shown in FIG. 1D, the releasable supporting sheet 111 that has thewiring pattern 113 and the releasable supporting sheet 118 that has thewiring pattern 117 are put on the surfaces of the adhesive insulatorsheet 114. Then, predetermined heat and pressure are applied to thislaminated sheet by a heating and pressing machine (not shown) for apredetermined period. In this heating and pressing step, the adhesiveinsulator sheet 114 is compressed and hardened completely, and theconductive material 116 in the through holes are also compressed to ahigher density, resulting in higher conductivity between the wiringpattern 113 and the wiring pattern 117.

In the next step shown in FIG. 1E, the releasable supporting sheets 111,118 are removed. Consequently, a double-side PWB 119 is obtained thatincludes the wiring patterns 113, 117 embedded in the insulator to makeflat surfaces, and the conductive material 116 that forms interlayerconnections (i.e., via connections).

Instead of forming wiring a pattern previously on the releasablesupporting sheet 111 or 118, a thin releasable copper film may be formedon the releasable supporting sheet, the copper film may be transferredonto the surface of the adhesive insulator sheet in the heating andpressing step, and the copper film may be etched to make a wiringpattern. This method has merit in that it can make a fine patternstably, although it needs more steps. This method should be included inthe scope of the present invention.

The conductive material 116 is not always required to be a conductivepaste. For example, it can be solder balls, gold balls or other metals.

If the wiring patterns 113, 117 are formed by printing a conductivepaste, the releasable supporting sheet can be an insulator sheet such asa polyester instead of the conductive sheet such as a stainless steelsheet. The surface of the sheet may be processed to have good propertyfor releasing.

The adhesive insulator sheet 114 is not limited to the above-mentionedaramid-epoxy prepreg, i.e., the non-woven aramid sheet impregnated withan epoxy resin. It may be a glass-epoxy prepreg or other adhesiveinsulator made of polyester, polyimide or other sheet coated with anadhesive or glue.

According to the present invention, double-side or multilayer PWBs canbe produced easily, which have fine patterns with a line width andspacing of 30 microns, and via-connections of the wiring patterns thatare formed at the same time as the transferring of the wiring patterns.

In FIG. 1, the width of the wiring pattern 113 (via pad) at thevia-connection point is not always larger than the diameter of thethrough hole 115. Registration of the fine wiring pattern with the viahole may be easier by making the width of the wiring pattern smallerthan the diameter of the through hole 115. This reason is illustrated inFIGS. 2A and 2B. FIG. 2A shows wiring patterns 201 on through holes 203filled with the conductive material. The width of the wiring pattern 201is smaller than the diameter of the through hole 203. The wiring pattern201 also serves as a via pad.

Since the method for producing PWBs according of the present inventionforms wiring patterns by transferring without etching step, theabove-mentioned configuration with smaller pattern width than the holediameter is possible. In the case of FIG. 2A, even if the wiring pattern201 is shifted to the position 202 shown in the broken line, electricconnection between them is still obtained.

On the contrary, if the wiring pattern is formed by etching of a copperfoil instead of transferring, the via hole (through hole) should becovered with a via pad as shown in FIG. 2B. Otherwise the conductivematerial in the via hole may be eroded by an etchant. In FIG. 2B, if thevia pad 201 is shifted to the position 202 shown with the broken line,it may contact with the neighboring via pad and make a short circuit.Therefore the allowable deviation S' in this case is smaller than that Sin FIG. 2A.

Since the width of the wiring pattern can be smaller than the diameterof the via hole, registration of patterns is easy in the productionprocess, and thus PWBs according to the present invention can beproduced inexpensively.

Second Embodiment

FIGS. 3A–3C are cross sections showing production steps of PWBsaccording to a second embodiment of the present invention.

In FIG. 3A, numeral 319 is a double-side PWB prepared for this step,which includes double-side printed wiring patterns 321 connectedelectrically with each other with via holes 320. On one side of the PWBan adhesive insulator sheet 314 is placed, which has through holesfilled with a conductive material 316 as explained in the firstembodiment (e.g., an aramid-epoxy prepreg). Similarly, on another sideof the PWB an adhesive insulator sheet 314 a is placed, which hasthrough holes 315 a filled with a conductive material 316 a. The twoadhesive insulator sheets 314, 314 a are made of the same materials buthave different positions for the through holes 315, 315 a in accordancewith the corresponding wiring patterns.

Then, a first releasable supporting sheet 323 whose surface is providedwith a first wiring pattern 322 is placed on the outer surface of theadhesive insulator sheet 314. Similarly, a second releasable supportingsheet 325 whose surface is provided with a second wiring pattern 324 isplaced on the outer surface of the adhesive insulator sheet 314 a.

As shown in FIG. 3B, a predetermined heat and pressure are applied onthe sides for a predetermined period by a vacuum press machine (notshown) as illustrated in FIG. 3B. For example, if an aramid-epoxyprepreg is used, the laminated sheet is kept at 30 Kg/cm² and 180degrees Celsius for one hour. Thus, the adhesive insulator sheets 314,314 a and the conductive material 316, 316 a in the through holes 315,315 a are compressed and hardened completely. The first wiring pattern322 and one of wiring patterns 321 of the double-side wiring PWB 319 areconnected electrically with the conductive material 316. Similarly, thesecond wiring pattern 324 and the other wiring pattern 321 of thedouble-side wiring PWB 319 are connected electrically with theconductive material 316 a.

In the next step shown in FIG. 3C, the first and second releasablesheets 322, 324 are removed. As a result, a multilayer PWB 326 isobtained, which has a four-layer wiring pattern including the first andsecond wiring patterns 322, 324 that are flush with the surfaces of theinsulator.

In the above explained embodiment, the double-side PWB 319 as a core canbe previously produced in accordance with the first embodiment.Alternatively, it may be a double-side or multilayer PWB made of apaper-phenol or a glass-epoxy in accordance with a conventional method.In this case, double-side or multilayer PWBs with a fine pattern (e.g.,30 microns of line width and space) at least on one side can be producedat a low cost by using an inexpensive PWB produced by a conventionalmethod as a core.

Third Embodiment

FIG. 4 is a cross section of a releasable supporting sheet used forproducing a PWB according to a third embodiment of the presentinvention. Two-layer wiring patterns are formed previously on thereleasable supporting sheet in this embodiment. On the surface of thereleasable supporting sheet 431, a first layer of wiring pattern 432 isformed, on which an insulator layer 433 is formed by coating orprinting. The insulator layer material is preferably an organic materialsuch as a photosensitive epoxy resin or polyimide resin.

Then, via holes 434 are perforated at predetermined positions of theinsulator layer 433 by a laser beam machine or etching, conductive pathsare formed inside the via holes, and a second layer of wiring pattern436 is formed on the surface of the insulator layer 433. Thus, thereleasable supporting sheet 431 having two-layer wiring patterns isprepared. The transfer step and other steps following theabove-mentioned step are the same as those in the first or secondembodiment. The supporting sheet or other members are made of the samematerials as in the first or second embodiment.

Three or more layer wiring patterns can be formed by a build-up methodusing the releasable supporting sheet 431 with two-layer wiring patternmentioned above. By using such a releasable supporting sheet withmultilayer wiring patterns, multilayer wiring patterns can betransferred onto the surface of the adhesive insulator sheet at onetime, so that inexpensive multilayer PWBs can be obtained.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The embodimentsdisclosed in this application are to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, all changes that come within the meaning and range ofequivalency of the claims are intended to be embraced therein.

1. A method for producing printed wiring boards, comprising the steps of: forming a first wiring pattern on a surface of a first releasable supporting sheet; forming a second wiring pattern on a surface of a second releasable supporting sheet; placing the first releasable supporting sheet on one surface of a first adhesive insulator sheet having holes filled with a first conductive material, and embedding the first wiring pattern in the first adhesive insulator sheet so that the first wiring pattern is connected with the first conductive material; placing the second releasable supporting sheet on the other surface of the first adhesive insulator sheet, and embedding the second wiring pattern in the first adhesive insulator sheet so that the second wiring pattern is connected with the first conductive material; and removing the first and second releasable supporting sheets from the first adhesive insulator sheet.
 2. The method according to claim 1, comprising the steps of: forming a third wiring pattern on a surface of a third releasable supporting sheet; laminating a second adhesive insulator sheet having holes filled with a second conductive material on the first adhesive insulator sheet; placing the third releasable supporting sheet on a surface of the second adhesive insulator sheet, and embedding the third wiring pattern in the second adhesive insulator sheet so that the third wiring pattern is connected with the second conductive material; and removing the third releasable supporting sheet from the second adhesive insulator sheet.
 3. the method according to claim 1, wherein the first conductive material is formed of a conductive paste.
 4. The method according to claim 1, wherein first releasable supporting sheet has conductivity and the step of forming the first wiring pattern on the surface of the first releasable supporting sheet is a step of forming the first wiring pattern by plating through a wiring pattern resist formed on the surface of the first releasable supporting sheet.
 5. The method according to claim 1, wherein the step of forming the first wiring pattern on the surface of the first releasable supporting sheet is a step of forming the first wiring pattern by printing a conductive paste on the surface of the first releasable supporting sheet.
 6. The method according to claim 1, wherein the step of forming the first wiring pattern on the surface of the first releasable supporting sheet comprises the steps of; forming a wiring pattern on the first releasable supporting sheet; forming an insulator layer on the wiring pattern; forming via holes at predetermined positions of the insulator layer; and forming the first wiring pattern on an upper surface of the insulating layer.
 7. The method according to claim 1, wherein the first adhesive insulator sheet is in a semi-hardened state.
 8. The method according to claim 1, wherein the first adhesive insulator sheet is formed of a porous base material.
 9. The material according to claim 1, wherein the first adhesive insulator sheet is hardened by application of heat and pressure, the first and second releasable supporting sheets are removed from the first adhesive insulator sheet after hardening.
 10. The method according to claim 1, wherein the adhesive insulator sheet is formed of a polyester or polyimide sheet coated with an adhesive or glue, or a glass-epoxy prepreg.
 11. The method according to claim 1, wherein the step of removing the releasable supporting sheets comprises removing the releasable supporting sheets by peeling or etching. 